For someone that's never played K2, could you give a detailed rundown of what this mod does compared to vanilla? All I'm seeing (if reading correctly) on the main page is how the added features differ from K2.

Good idea to make a description for not familiar with Krastorio2. I'll do it.

TWO aspects added in game:
I.
All nests and worms are surrounded by special cover on the ground - creep. It slows down you (35% of speed), make vehicles not well controlled (shouldn't reduce speed, but difficult to accelerate/brake and turn). Creep footprint grows with evolution.
Kill nests and worms first to collect this cover from ground with special tool (available in shortcut and via Alt+C - controls can be changed) like deconstruction planner.

If one builds something on this creep, acid damage (corrosion) is applied once per second (10%-20% actual building's health) until 5-10 health points left (so building will not be destroyed by creep, but gives much higher chance of destruction by biters/worms ). To stop damage one needs to collect creep OR take building back into inventory.
This changes early/early-mid game with installing rows of turrets closer and closer to nests - player needs to repair turrets and do everything faster. Or select other tactics and keep turrets out of creep (as a back point for protection). I'll make a short demo video for it soon.
The key idea is that creep can't be gathered before its hosts (nearby worms and nests) are killed. So player must be faster acting or use other tactics (rocket launcher, grenades, etc).
For endgame (artillery, rocket launcher, nukes) it doesn't make sense in tactics. For now. Maybe I'll add some, but not sure about it, because playing together with Rampant mod ("New Enemies" must be turned On) it is already fun enough, as biters expand in very different manner with that mode, and there are many factions of biters who can't be easily killed with one type of ammo (except maybe with nukes, but a lot of nukes).

II. Collected creep is biomass.
Biomass is added as an ingredient for biters research pack, which are used in production of military science packs. So to gather for example 600 military science packs for some middle game technology you need to gather 4200 biomass (4-6 average biter's "bases" with nests and worms). With evolution growth and distance from starting position nests are settled in bigger packs with bigger amounts of creep, therefore 2 bases can be enough (but you won't be able to overpower them until tank or armor MK-2 and other technologies unlocked and used).
With Rampant mod things are much more complex.

Together with Rampant and it's counter-part mod Rampant Arsenal things become much more complex in terms of biters defenses and biomass economics.

I hope that gives some understanding. I made this mod for myself, because I can't play Krastorio2 - for me it breaks balance a lot despite brilliant ideas introduced (creep, radioactivity, new resources, extended endgame). And I don't like science cards concept (despite nice icons graphics made for it).
In the perfect world I'd also prefer a separate mod for radioactivity (it's easier to do, than was taking creep out), separate mod with interesting endgame adventures (but without balance-breaking conversion of everything into dark matter and back).

That helps a lot, thank you!

This all sounds very interesting. Sounds like biomass is (kind of) taking the place of the old alien artifacts they used to have us collect for advanced research. Definitely makes sense to apply it to military, too.

One question:

playing together with Rampant mod ("New Enemies" must be turned On)

So, must this be played with Rampant new enemies? I use and like Rampant, but don't use the new enemies. It's more than what I want to deal with at this time.

Also:

If one builds something on this creep, acid damage (corrosion) is applied once per second (10%-20% actual building's health) until 5-10 health points left (so building will not be destroyed by creep, but gives much higher chance of destruction by biters/worms ). To stop damage one needs to collect creep OR take building back into inventory.
This changes early/early-mid game with installing rows of turrets closer and closer to nests - player needs to repair turrets and do everything faster. Or select other tactics and keep turrets out of creep (as a back point for protection). I'll make a short demo video for it soon.

You don't need to make a video if you don't want to. I think your explanation is pretty clear.

Sounds like biomass is (kind of) taking the place of the old alien artifacts they used to have us collect for advanced research. Definitely makes sense to apply it to military, too.

Yes, it's the best reincarnation of alien artifacts. Much better than original in ancient Vanilla and restored now in other mods. But it conflicts with "full automation beauty" concept most of Factorio players come for.

So, must this be played with Rampant new enemies? I use and like Rampant, but don't use the new enemies. It's more than what I want to deal with at this time.

No, it's not a must. You can play without "new enemies". But you'll win vs them technologically very easy - laser and common flame turret do the thing. Rampant makes them much smarter: no dragon teeth, labyrinth of death and other vanilla cheats, but still biters become very weak after all pre-space tiers of laser and fire damage are obtained. "New enemies" make them horrible all the way. You can set max tier = 5, turn off Inferno faction and you'll be ok. No need to set max tier = 10 in first "new enemies" game.

Are you looking for the "full automation beauty" concept? If you are then creating an oil like resource that would generate biomass would be a solution that has some automation components.

EDIT: Nevermind, read front page closer

I liked everything I read except the "harvest biomass" part. Nice work.